The cause is too much weight on the hands, but most so far have suggested a "gear" fix, a new stem, new bike, better gloves.

Leave everything on your bike exactly as it is and try to alter your upper body position. When you ride, you should have very little weight on your hands.

Try this. When you are on your bike, keep yout butt where it is, don't let it slide back, and push away from the bars with your arms, rotating your hips more towards the vertical and arching your back. Push back until you feel little weight on your hands. This is the new position to train in. It will feel awkward at first, but repeatedly push back as you train to maintian the position. Stretch and do sit up to keep your core strong and flexible.